The present invention is concerned with an optical communication system, in particular, but not exclusively, with an optical communication system including interconnected optical communication rings. Moreover, the invention also relates to a method of operating such a system. Furthermore, the invention also relates to an interface for use in the system.
Conventional optical communication systems comprise nodes interconnected by optical fibre waveguides. Communication traffic is communicated between the nodes by conveying optical radiation through the waveguides, the radiation being modulated by the communication traffic. Optical radiation in the context of the present invention is defined as electromagnetic radiation within a free-space wavelength range from 560 nm to 2000 nm, although a free-space wavelength of substantially 1550 nm is a preferred part of this range.
Each node is operable to convert modulated radiation received thereat into corresponding electrical signals. Moreover, each node is further operable to convert electrical signals thereat into corresponding modulated optical radiation and emit the radiation into waveguides connected thereto. Electrical signals can be input and output from the nodes if required, for example to provide signals to clients connected to the nodes and to receive signals from the clients for transmission within the systems.
Examples of conventional optical communication systems including interfaces are to be found in the prior art and will now be described.
In a PCT published patent application PCT/SE96/01265, an optical cross-connect node architecture is described which is capable of interfacing a plurality of optical fibre input and output links, each link itself providing a plurality of wavelength division multiplexed (WDM) channels. In a first embodiment of the architecture elucidated, the input links are connected to an optical coupler. Pairs of tunable optical filters and optical wavelength converters are each connected to an output port of the optical coupler and perform wavelength channel routing and switching in the wavelength domain, namely without the need for any optical space switch. In a second embodiment of the architecture elucidated, an additional input wavelength converter is connected to each input fibre link to convert the plurality of wavelength channels on each link to different non-interfering wavelengths. Such conversion prevents wavelength contention in the optical coupler to which the input wavelength converters are connected. New fibre links may be added in modular fashion without significant impact on the pre-existing optical cross-connect structure. Similarly, new wavelength channels may also be multiplexed onto existing fibres to provide wavelength modularity without having to reconfigure the node.
In a U.S. Pat. No. 5,726,785, a multiplexer for use in optical telecommunications is described. The multiplexer is operable to add WDM optical radiation components to a group of existing WDM optical radiation components, the existing components having wavelengths belonging to a group of wavelengths λ1 to λN. Moreover, the multiplexer is also operable to drop from the group of existing components at least one WDM optical component having a given wavelength chosen from within the aforementioned wavelength group. The multiplexer comprises at least one circulator having an optical input port for receiving the group of existing WDM components and an optical output port, and an optical selection means coupled on one side thereof to the circulator. The selection means comprises at least one photoinduced Bragg grating, the grating being associated with a corresponding wavelength. The grating can selectively switched between a first state and a second state. In the first state, the grating reflects a WDM radiation component having the given wavelength and transmits WDM radiation components having a wavelength different from the given wavelength. In the second state, the grating transmits all WDM radiation components. Means for controlling the selection means for switching it between the first and second states is provided. Moreover, the selection means cooperates with each optical circulator for adding and dropping the one or more WDM radiation components. Thus, U.S. Pat. No. 5,726,785 is primarily addressed at the problem of selectively routing WDM radiation components.
In a European patent application no. EP 0 862 071 A1, an optical branching device and a method of optical transmission is described. In the application, it is identified that, in an optical transmission system comprising an optical branching device for routing WDM radiation components, the number of radiation components can change or the intensity of the components can change for a number of reasons during operation. Such a system can include an output optical amplifier for compensating for such change to achieve a substantially constant output power. However, as the number of radiation components changes, the power of each of the remaining components will be modified by such compensation which often represents a departure from optimum device operating conditions. Thus, there is provided in the patent application an optical branching device comprising two optical circulators and plurality of optical fibre gratings arranged in series. The gratings are connected between the two optical circulators. Each grating is operable to reflect one or more radiation components having a wavelength different to the radiation components being transmitted. If a malfunction or similar disruption occurs, the device is operable to divert one or more radiation components different from the transmitted radiation components from a first node including the device to an auxiliary node remote from the terminal node, thus maintaining the radiation components transmitted through the first node to a prescribed power level and ensuring that, for example, the optical amplifier is operated at its optimum function point. Thus, the patent application EP 0 862 071 A1 is concerned with a problem of selectively diverting WDM radiation components to maintain optical working power levels when the number of components is dynamically changing in operation.
In another European patent application no. EP 0 926 853 A2, there is described a wavelength-selective add-drop multiplexer for adding and/or dropping spectral components from a WDM optical signal. 1×1 and 2×2 optical switches are included in the multiplexer; the switches are either used alone or in conjunction with other optical elements to separate WDM spectral components for dropping from other spectral components. The switches are disclosed as being micro-electromechanical actuators for positioning a reflective device into or out of a path of a spectral component for controlling its routing in the multiplexer. Thus, the patent application EP 0 926 853 A2 is concerned with addressing the problem of selective WDM spectral component routing within an optical communication system.
In a United Kingdom patent application GB 2 321 809 A, an add/drop multiplexer is described for coupling trunk and branch optical fibre waveguides in a WDM optical network. The multiplexer selectively feeds particular carrier wavelengths λ1, λ2 from first and second trunk inputs to first, second and third trunk and branch outputs, and from a third branch input to the first and second branch outputs. WDM spectral components are selected by, for example, fibre Bragg gratings acting as reflecting filters, and routed via circulators. During such routing, the spectral components may be amplified by pumped bi-directional doped optical fibre amplifiers. Thus, the patent application GB 2 321 809 A is concerned with a problem of WDM spectral component routing and amplification.
In another European patent no. EP 0 720 408 A2, there is described a tunable add/drop optical filter providing arbitrary channel arrangements between two input WDM signals and two output WDM signals. The filter comprises two N-port wavelength grating routers (WGRs) connected by 2×2 optical switches in each WGR port branch. The switches can, for example, be opto-mechanical switches capable of switching in 50 ms. Thus, the patent application EP 0 720 408 A2 is concerned with selective routing of WDM signal spectral components.
In a scientific publication “Analysis of Hot-Potato Optical Networks with Wavelength Conversion”, Bononi and Castanon, Journal of Lightwave Technology, Vol. 17, No. 4 April 1999, there is described a general analysis of WDM spectral component routing. However, practical hardware for performing such routing is not disclosed in the publication.
In the aforementioned conventional systems, the optical radiation propagating therein typically has a wavelength in the order of 1550 nm. This wavelength corresponds to a radiation frequency of around 200 THz and theoretically offers a maximum communication bandwidth in the order of 100 THz taking into consideration the Nyquist criterion, namely that carrier radiation must have a carrier frequency at least twice that of the highest frequency of a signal modulated onto the carrier radiation to circumvent aliasing and information loss. It is conventional practice, as elucidated in the foregoing, to partition radiation propagating in the conventional systems into wavebands, each waveband having associated therewith information-bearing radiation; such partitioning is known as WDM.
In practice, converting optical radiation into corresponding electrical signals at each node in conventional systems imposes a severe limitation on the communication bandwidth which can theoretically be provided by these systems. Such a limitation of bandwidth represents a serious first problem for the conventional systems.
In order to address the first problem, there has recently been theoretical studies concerning optical soliton wave propagation within optical systems. Such soliton waves are capable of propagating over relatively long distances through optical waveguides whilst suffering negligible dispersion and loss. It is not practicable to exploit soliton wave propagation in conventional optical communication systems on account of frequent conversions between modulated optical radiation and corresponding electrical signals which occur in such systems; these conversions negate any potential benefits from exploiting soliton propagation.
The inventors have appreciated that it is highly desirable in an optical communication system to perform as much processing as possible within the optical domain to address the first problem and only convert between optical radiation and corresponding electrical signals when absolutely necessary for performing specialist functions, for example signal regeneration. Regeneration is required for at least partially reversing the effects of dispersion which arise when optical signals are transmitted through relatively long lengths of optical fibre waveguide, for example 100 km lengths of optical fibre. The inventors have therefore devised a regenerative interface for a communication system, the interface capable of providing flexible re-routing of communication traffic whilst performing as much optical processing as possible; attempts so far in the prior art to provide an all-optical communication system have been frustrated by technical difficulties, particularly with regard to achieving all-optical reconfigurable radiation routing.
Moreover, the inventors have appreciated a second problem in conventional communication systems regarding bandwidth limitation arising from inefficient use of system wavebands. The inventors have realized that it is also desirable to be able to redistribute communication traffic within the system between wavebands to ensure that the system is capable of providing its full communication bandwidth in operation when heavily loaded with communication traffic.
Furthermore, the inventors have appreciated a third problem that it is highly desirable to be able to redistribute communication traffic within the system without having to convert information-bearing radiation into corresponding electrical signals which can represent a bandwidth limitation.